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How do MRI and PET work?

How does MRI work?

Magnetic resonance imaging (MRI) is based on the magnetic properties of atomic nuclei, usually of the hydrogen nucleus, which is found in abundance as an element in water in the human body.

When a person is in an MRI machine, certain atomic nuclei in his/her body are aligned like small bar magnets. Scientists specifically "disturb" this order using high-frequency radio waves, thus making the atomic nuclei "spin".

When the radio signal is turned off, the nuclei gradually return to their initial position. The energy absorbed beforehand is released in the form of radio waves during this process. Using the strength and duration of the spinning motion, researchers are able to differentiate between the different types of tissue and thus create images of the brain structures with a resolution of less than one millimetre.

The behaviour of atomic nuclei is different depending on the type of tissue. The stronger the magnetic field inside the bore of the MRI machine, the clearer the differences between different types of tissue – anatomical differences become more evident.

A certain kind of MRT – functional magnetic resonance tomography (fMRT) – provides information on the brain regions that are supplied with more blood, for example, when patients solve problems or look at images. PET provides similar results, however, more selectively and with a sensitivity that is 1000 times higher. PET produces images of metabolic processes, brain activities and receptor occupancies.

How does PET work?

In positron emission tomography (PET), a weak radioactive substance – known as a radiotracer – generates the signal. Patients receive an injection of this substance before measurements are taken. These radiotracers travel to the brain via the bloodstream and accumulate in specific cells depending on the type of tracer: sugar molecules, for example, accumulate where an increased energy demand exists and neurotransmitters at the corresponding receptors.

The decay products of the radioactive substance provide information on the processes investigated. When radioactive atoms decay spontaneously in the tracer, positrons are produced. When these positrons hit electrons, a measurable signal is generated (two gamma rays). With a resolution of only a few millimetres, they show where the radiotracer was involved in molecular processes.

In this way, brain activities and metabolic processes can be observed, as can tumour tissue or receptors that are responsible for the communication between brain cells.

In order to investigate such processes in the brain, tailor-made radiotracer must be available. Jülich researchers at the Institute of Nuclear Chemistry are therefore developing and testing these.